The present invention relates to a semiconductor laser device, and more specifically, the present invention relates to a shape of a sealing resin layer for sealing a laser diode (hereinafter simply referred to as an LD) element of the semiconductor laser device.
Semiconductor laser devices are used in various optical devices and apparatuses including optical disks such as compact disks (hereinafter simply referred to as CD), laser beam printers, etc.
As the semiconductor laser devices, a can-type semiconductor laser device is well-known. FIGS. 5(a) and 5(b) schematically illustrat a configuration and mounting state of the can-type semiconductor laser device. As shown in the partly perspective view of FIG. 5(a), an LD element comprising an LD chip 1 and a sub-mount layer 2 as a heat radiator plate, is soldered to a radiator block 4 protruding upward from a stem 3. A cap 6 having a glass window 5 on its top, for covering and for protecting the LD chip 1, is soldered to the stem 3. Figure 5(b) is a sectional view showing installation of the semiconductor laser device on a part 7 of an optical apparatus. A trench 8 is provided between the cap 6 and the part 7 of the apparatus. In FIG. 5(b), a laser beam 9 is emitted in a direction indicated by an arrow.
FIG. 6(a) is a front plan view of the can-type laser diode device and FIG. 6(b) is a sectional view taken along A--A of FIG. 6(a). In FIGS. 6(a) and 6(b), the same parts with those in FIGS. 5(a) and 5(b) are designated by the same reference numerals. As shown in FIG. 6(a), the laser beam emitting point of the laser diode device should be kept at a predetermined point. The LD chip 1 is arranged so that its laser beam emitting point is positioned at a cross point 10 of a central axis of the stem 3 and the glass window 5, X-axis vertical to a major surface of the radiator body 4, and Y-axis parallel to the major surface of the radiator body 4. Also, arrangement of the sub-mount 2 and the radiator body 4 is determined. As shown in FIG. 5(b), the semiconductor laser device is installed on the apparatus usually by inserting the cap 6 into the trench 8 and by adhesion or pressurized-bonding of a flange portion 3a of the stem 3 to the apparatus. The location of the laser beam emitting point is defined by the outer periphery and upper face of the flange portion 3a. The shape and dimensions of the semiconductor laser device including the flange portion 3a have been standardized so as to avoid changes of design and parts of the apparatuses into which the semiconductor laser device is incorporated. For example, an outer diameter of the most popular semiconductor laser device with low output power of 3 to 5 mW for CDs is specified at 5.6 mm, and an outer diameter of a high output power semiconductor laser device is specified at 9 mm.
A semiconductor laser device to be developed should be one which can be handled in the same way as the conventional semiconductor laser devices for avoiding the aforementioned changes of design and parts of the apparatuses. That is, the new semiconductor laser device should be the same as the conventional one in its installation mechanism and location of the laser beam emitting point. In addition, the new semiconductor laser device should be cheaper than the conventional one. Recently, a resin-sealed-type (mold-type) semiconductor laser device has been developed which is cheaper than the conventional can-type semiconductor laser device. The mold-type semiconductor laser device has more freedom in designing its shape and dimensions than the can-type one.
FIG. 7 is a perspective view showing the aforementioned mold-type semiconductor laser device disclosed in Japanese Laid Open Patent Publication No. Hei. 2-125687 and in Japanese Laid Open Patent Publication No. Hei. 2-125688. In FIG. 7, the mold-type semiconductor laser device comprises an LD chip 1 mounted on a sub-mount layer 2, and a cylindrical sealing resin layer 11, made, for example, of transparent epoxy resin, surrounding the LD chip 1 and the sub-mount layer 2. The sealing resin layer 11 comprises a cylindrical flange portion 11a corresponding to the flange portion 3a of the stem 3 in the can-type semiconductor laser device. The mold-type semiconductor laser device is driven via connecting leads 12 and gold wires 13. A resin-mold-type device has been applied to the light sources with low beam density per unit area such as light emitting diodes (LEDs).
FIG. 8 is a schematic sectional view showing the structure of the LD chip 1. In FIG. 8, the LD chip 1 has a double hetero (DH) structure which comprises an n-type GaAs substrate 14, an n-type AlGaAs clad layer 15, a GaAs active layer 16, a p-type clad layer 17, and a p-type cap layer 18. A top surface of the p-type cap layer 18 (major face of the LD chip) is covered with an electrode 19 and the bottom surface of the GaAs substrate 14 is covered with a back electrode 20.
FIG. 9 is a sectional view of the LD chip 1 taken along A--A of FIG. 8. In FIG. 9, the same parts as those in FIG. 8 are designated by the same reference numerals. As shown in FIG. 9, the LD chip 1 is further comprised of protective layers 22 formed on end faces 21 from which the laser beam 9 is emitted. The protective layers 22 are formed for preventing the end faces 21 from breakdown. The protective layer 22 is made, for example, of silicone which shows a low optical absorption coefficient in the wavelength range of the laser beam 9 and high thermal endurance. The protective layers 22 prevent the properties of the semiconductor laser from deterioration caused by optical damage of the sealing resin layer 11. The protective layers 22 provided between the end faces 21 of the LD chip 1 and the sealing resin layer 11, attenuate laser beam density in the sealing resin layer 11 and prevent the sealing epoxy layer 11 from being optically damaged by the laser beam 9.
The resin-mold-type devices described above are well suited for cost reduction and enlarging design freedom. The resin-mold-type device is applicable also to laser diodes with high beam density. The resin-mold-type semiconductor laser device as shown in FIG. 7, which has the same shape with that of the conventional can-type semiconductor laser device, is well suited for installing on the optical apparatuses.
FIGS. 10(a) and 10(b) show a main portion of the resin-mold-type semiconductor laser device of FIG. 7, in which FIG. 10(a) is a front plan view and FIG. 10(b) is a sectional view taken along A--A of FIG. 10(a). In FIGS. 10(a) and 10(b), the same parts as those in FIG. 7 are designated by the same reference numerals. In FIG. 10(a), the LD chip 1 is positioned in the center of the sealing resin layer 11 at a cross point 10 of X-axis (perpendicular to the major face of the LD chip 1) and Y-axis (parallel to the major face of the LD chip 1) similarly as in the can-type semiconductor laser device (see FIG. 6(a)). A center 23 of the connecting leads 12 is displaced by an offset distance .DELTA.X.sub.off inevitably determined by the position of the LD chip 1, the thickness of the sub-mount layer 2 and the thickness of the connecting lead 12. Explanations with reference to FIG. 10(b) will be omitted for avoiding duplication.
The structure of the semiconductor laser device shown in FIGS. 10(a) and 10(b) accompanies two major problems described below.
(1) The laser beam emitting point displaces by temperature rise of the resin around the LD chip 1 caused by power supply to the LD chip 1 or by temperature rise of the environment. PA1 (2) The sealing resin layer 11 peals off from the protective layer 22. This pealing-off causes deterioration of beam radiation characteristics (far field pattern characteristics: FFP characteristics).
Now, the problem (1) will be explained more in detail. FIG. 11 shows an example of displacement of the laser beam emitting point. FIG. 11 is a graph showing the relation between the displacement of the laser beam emitting point along X-direction shown in FIG. 10(a) and the operation time of the semiconductor laser device. In FIG. 11, a single-dotted chain line represents the displacement in the cylindrical resin-mold-type semiconductor laser device and a solid line represents the displacement in a flat resin-mold-type semiconductor laser device described later.
As FIG. 11 shows, an emitting point of the laser beam is displaced along X-direction when the semiconductor laser device of FIG. 10(a) is driven with a current of 50 mA at a room temperature. As shown in FIG. 11, the laser beam emitting point is displaced by 0.5 .mu.m in -X-direction (+X is taken on the side of the LD chip 1 and -X on the side of the connecting leads 12) in 2 min after the laser diode is turned on and returns to the original position in 2 min after the laser diode is turned off. When this semiconductor laser device is incorporated, for example, into an optical pickup for CDs, trouble is caused in the CD device immediately after the semiconductor laser is turned on or by temperature change of the environment.
It has been found that the displacement of the laser beam emitting point is caused by the displacement of the connecting leads 12 in X-direction by thermal expansion of the sealing resin layer 11 which is caused by heat generated from the LD chip 1 or temperature change of the environment. This displacement of the connecting leads 12 by thermal expansion of the sealing resin layer is closely related to the offset 24 of .DELTA.X.sub.off of the connecting leads 12 from the center 10 of the sealing resin layer 11 shown in FIG. 10(a). However, the offset .DELTA.X.sub.off affects the displacement of the laser beam emitting point via the thermal expansion of the sealing resin layer near the LD chip 1. The sealing resin layer away from the LD chip 1, for example, the flange portion 11a of FIG. 10(a) does not cause the displacement of the laser beam emitting point.
It is necessary, for avoiding the displacement of the laser beam emitting point, to form the sealing resin layer 11 so that the connecting leads 12 are positioned at the center of symmetry of the sealing resin layer 11. If one considers a cross section of a central connecting lead parallel to the front laser beam emitting face of the LD chip 1, it is enough to form the sealing resin layer 11 symmetrically in volume at least nearby the main portion of the LD chip 1 with respect to a center line (parallel to Y-axis) of the central connecting lead 12. That is, it is necessary to form the sealing resin layer 11 so that the center 10 of the sealing resin layer 11 may coincide with the center 23 of the central connecting lead 12, though it is not always necessary to form the sealing resin layer 11 symmetrically in its portions away from the LD chip 1 or the connecting leads 12.
The pealing-off problem of (2) is practically solved by forming the sealing resin layer 11 symmetrical with respect to the connecting leads 12 as described above and in a thin flat plate.
FIG. 12 is a schematic perspective view showing a flat resin-mold-type semiconductor laser device disclosed in Japanese Laid Open Patent Publication No. Hei. 2-125687. This semiconductor laser device facilitates relieving stress caused by thermal expansion of the resin by reducing the volume of the resin which covers around the protective layer 22 and by equalizing the sealant resin volume around the LD chip 1.
Table 1 shows comparison results of electrical and optical properties measured at predetermined cycles in a heat cycle test conducted on a test specimen of a cylindrical resin-mold-type semiconductor laser device as shown in FIG. 7 and a test specimen of a thin flat resin-mold-type semiconductor laser device as shown in FIG. 12. In both sample semiconductor laser devices, the protective layers 22 were made of gummy organosilicon resin containing dimethylsiloxane as its main component. The heat cycle test repeated one heat cycle consisted of heating at 85.degree. C. for 30 min, rapid cooling down to -40.degree. C., and keeping at -40.degree. C. for 30 min followed by return to 85.degree. C.
TABLE 1 ______________________________________ Shape of Specimen sealing Content of Numbers of heat cycle No. resin layer fault 100 200 300 400 ______________________________________ 1 Cylindrical FFP deterio- 0 5 8 12 ration (%) Fault other 0 0 0 0 than FFP det- erioration (%) 2 Flat FEP deterio- 0 0 0 0 ration (%) Fault other 0 0 0 0 than FEP det- erioration (%) ______________________________________
As is apparent from Table 1, the far field pattern (FFP) of the laser beam from the cylindrical resin-mold-type semiconductor laser device of FIG. 7 deteriorated during the heat cycle test. However, any fault was not observed on the thin flat resin-mold-type semiconductor laser device of FIG. 12. The observed far field pattern (FFP) deterioration was caused by pealing-off between the protective layer 22 and the sealing resin layer 11. Thus, the thin flat resin-mold-type semiconductor laser device prevents pealing-off of the resin layers.
In this thin flat resin-mold-type semiconductor laser device, any displacement of the laser beam emitting point did not occur as shown in FIG. 11.
As described above, the resin-mold-type semiconductor laser device is well suited for cost reduction irrespective of whether its sealing resin layer is cylindrical or thin and fat. Though the cylindrical resin-mold-type semiconductor laser device is easily mounted on an apparatus as the can-type semiconductor laser device through the flange portion formed as a part of the sealing resin layer, the sealing resin layer is bulky and asymmetric with respect to the center line of the connecting lead. This asymmetry generates stress by temperature change and causes FFP deterioration by displacement of the laser beam emitting point. The thin flat resin-mold-type semiconductor laser device, the sealing resin layer of which is small in volume and symmetric with respect to the center line of the connecting lead, solves the problem described above. However the conventional thin flat resin-mold-type semiconductor laser device is not mounted on an apparatus in the same way as the can-type semiconductor laser device, since the conventional thin flat resin-mold-type semiconductor laser device lacks the flange portion. Thus, the cylindrical and the thin flat resin-mold-type semiconductor laser devices accompany the problems which are incompatible with each other.